534 



this point of view tlie problem could be solved without reference 

 to medium (2) by introducing a boundary condition in (he medium 



(1) which is to express the fact that the flow — ~( ~7~ I is spent 



in supplying the rpiantity H to new regions of the material having 



initially H := H, and converted to H=Hc. The length of the region 



(/.r,. 

 converted per second is -— and thus the boundary condition is: 



The problem can be also solved from this point of view. 



This direct solution for the case (i, = oo naturally leads to the 

 same result which we have obtained by passing to the limit of 

 (?,—*• 00. It may be, however, that other problems may be more 

 easily solved for the case of (J, ^ cc by this method than bj^ passing 

 to the limit. 



Case 11. Penetration of supraconductivity into a non-supracon- 

 ductor. 



We next pass to the case of a material in which the supra-con- 

 ductivity has been destroyed by a magnetic field, we diminish the 

 field from the outside so as to reestablish supra-conductivity. The 

 supra-conductivity is reestablished first in the external layer of the 

 metal and propagates inward as time goes on. 



Fixing our attention again on Fig. 2 we suppose that just before 

 t=iO the magnetic field H has a uniform value H^ throughout 

 x^O and .r ^ 0. Tliis value ff, is greater than the critical field 

 He- At <^0 the value of H al the left of AB is dropped to 



TT, < H,. 



After the lapse of a lime t the boundary between the two con- 

 ducting states will have advanced a distance Xc- For x<^Xi. the 

 metal is microresidually conducting and /? = ;?,. For x'^Xc the 

 metal has its ordinary conductivity and (i^/i,. The expression for 

 H for .« <^ .XV will be written as //, . As in the first case we are 

 induced to try to satisfy our equations by expressions of the form -. 



H, = A, + B,& 





